This invention relates to an optoelectric converter.
In optical serial transmission of digital data, baseband binary data is used to modulate an electrical data signal which controls the state of a laser diode at the transmitting end of a fiber optic cable. An optoelectric converter at the receiving end of the fiber optic cable is used to convert the optical signal back to electrical form so that the binary data can be recovered.
Referring to FIG. 1, a conventional optoelectric converter includes a photodiode D10, which generates a very small current signal proportional to the received optical power, a transimpedance amplifier 12 for converting the current signal to a voltage signal, and a gain stage 13 having a fixed gain. The transimpedance amplifier and the gain stage are typically AC coupled. A comparator 14 compares the voltage signal provided by the gain stage 13 with a slice level Vslice for recovering the baseband data. The gain of the gain stage 13 is selected so that the voltage signal provided by the gain stage will be compatible with the comparator 14. Specifically, for accurate recovery of the data, it is necessary that the voltage range of the output signal of the gain stage 13 should bracket the slice level Vslice.
The laser diode at the transmitting end of the fiber optic cable is operated at two distinct non-zero current levels of the same polarity depending on the logic state of the baseband data. Correspondingly, the signal generated by the photodiode D10 has two distinct non-zero current levels of the same polarity. The transimpedance amplifier converts the two distinct non-zero current levels of the same polarity to two distinct non-zero voltage levels of the same polarity.
If the strings of consecutive 1""s and 0""s in the baseband data are very short and the time constants of the AC coupling capacitor and associated impedances are sufficiently long, the DC level of the voltage signal generated by the transimpedance amplifier and gain stage is preserved. Accordingly, the DC level of the output voltage signal of the transimpedance amplifier is non-zero and is typically such that the two voltage levels corresponding to the two non-zero current levels of the input current signal are both positive. The data recovery slice level is set approximately midway between the two positive voltage levels and the baseband data is recovered with a high degree of accuracy. If, however, the baseband data includes a long sequence of 1""s or 0""s, the coupling capacitor charges and the average signal level rises or falls, depending on whether the sequence is of 1""s or 0""s, and the range of the output signal may no longer bracket the data recovery slice level and the data cannot be recovered accurately.
The digital source data that is to be transmitted over a fiber optic cable may be composed of multi-bit words which are coded as a serial binary data stream for serial propagation. For efficient data propagation, it is desirable that the baseband data have zero DC content.
The SMPTE 259 standard for serial digital interface (SDI) signals and the SMPTE 292 standard for high definition serial digital interface (HDSDI) signals each prescribe a scheme for mapping 10-bit video data words to a serial binary data stream. The serial data stream has a data rate up to 1.5 Gb/s. SMPTE 259 and SMPTE 292 each prescribe a polynomial, or PN, scrambler, which functions well to generate baseband data having minimal DC content provided that the video data supplied to the scrambler is random, or nearly so. When the source of the video data is a camera, noise generated in the camera provides a sufficient degree of randomness. However, the content of some computer generated video data is not sufficiently random, and the PN scrambler can generate baseband data having very long strings of consecutive 1""s and 0""s in response to these so-called pathological signals.
In the case of a pathological signal, the period of the low frequency content of the voltage signal generated by the first stage of the transimpedance amplifier may exceed the time constants associated with the AC coupling capacitor. As a result, the DC level of the voltage signal generated by the transimpedance amplifier drifts up or down so that the voltage range of the signal no longer brackets the data recovery slice level. It is therefore necessary to restore the DC level of the data signal in order to achieve accurate data recovery.
It would be possible in principle to build a DC restorer using discrete components, but there are significant practical difficulties in using discrete components to build a DC restorer that is able to function with signals having data rates as high as 1.5 Gb/s. These practical difficulties could be avoided or reduced by use of a monolithic integrated circuit, but a DC restorer in the form of a monolithic integrated circuit is not available commercially as a separate product.
A cable equalizer is used to compensate for frequency-dependent attenuation of an electrical signal being propagated over a conductive cable. The conventional cable equalizer detects the amplitude of the signal at a selected time and compares this amplitude with a reference value, and uses the result of this comparison to control a frequency-dependent emphasizer or amplifier, so as to eliminate the difference between the measured value and the reference value.
Cable equalizers suitable for equalizing serial binary data signals having data rates up to 1.5 Gb/s are commercially available in the form of monolithic integrated circuits. For example, the type GS 9024 monolithic integrated circuit is suitable for equalizing SDI signals and the type GS 1504 monolithic integrated circuit is suitable for equalizing HDSDI signals. Each of these cable equalizers has an output stage which provides a DC restore function to ensure that the range of the output signal brackets the standard data recovery slice level; each also has a cable-length indicator output, which provides a signal indicating the length of cable through which the signal has passed.
It would not normally be necessary or desirable to equalize the signal generated by the transimpedance amplifier shown in FIG. 1, because the data signal has not been propagated over an electrically conductive cable, and equalization would tend to overequalize the data signal, distorting its zero crossings and wave shape.
According to the present invention there is provided an optoelectric converter for generating a binary electrical data signal from a binary optical data signal, comprising a semiconductor photodetector for receiving the optical data signal and generating an electrical current signal, an amplifier means receiving the current signal generated by the semiconductor photodetector and providing a voltage signal at an output of the amplifier means, an equalizer means which receives the voltage signal generated by the amplifier means and generates an equalizer output voltage signal, and a DC restore means which receives the equalizer output voltage signal and generates a DC restored voltage signal.